Generally speaking, puffer interrupters do not have the interrupting capacity of a multi-pressure circuit breaker (sometimes called a "double pressure interrupter"). In large part this is due to their inherent limitations. In a multi-pressure circuit breaker, a gas having a strong arc extinguishing capability (such as sulfur hexaflouride SF.sub.6) is continuously stored at high pressure. The pressure is released or valved out to the circuit breaker contacts only when interruption is to occur. Since the gas is stored at high pressure for use at a later time, a compressive means or gas compressor having a relatively low flow or compression rate can be used to prepare the device for operation. Accordingly, ample gas is available for interruption and arc extinguishing is good. However, with this type of circuit interrupter, the pressure generating apparatus is relatively complicated and the overall interrupter is relatively large in size; moreover, maintenance has often proved to be a problem. One such interrupter is illustrated in U.S. Pat. No. 2,783,338 issued to Beatty.
In contrast, a puffer interrupter (sometimes called a "single pressure puffer interrupter") does not store the extinguishing gas in a high pressure condition. Instead, a compressive mechanism, typically a piston and cylinder arrangement, is used to compress the gas to the required pressure just prior to circuit interruption. Fischer (U.S. Pat. No. 3,406,296) describes a typical device. Accordingly, a relatively large amount of energy is required to pressurize the gas to the required pressure and at a sufficiently high rate to perform the interruption within the time available. To minimize the loss of pressure or pressure drop after the gas is compressed, the compressive mechanism is located as close as possible to the arc site. Often, the piston used to compress the gas is moved by the same mechanism which moves one of the contacts across which the arc is drawn. From an efficiency standpoint, it is desirable that the smallest amount of gas should be pressurized to the minimum required pressure within the shortest time available. If more gas is compressed than is needed, or if the gas is pressurized to a higher pressure than what is necessary, the prime mover or mechanism used to operate the circuit breaker is not designed in the most efficient manner.
When a multi-pressure circuit breaker is used, it is relatively easy to stage the stored high pressure gas or to compress the gas to the minimum value necessary to achieve interruption. In a puffer interrupter, on the other hand, it is not easy to pressurize the gas to the required minimum pressure or to maintain this pressure at the desired minimum without achieving some overshoot. Any amount of overshoot is wasted work as far as the prime mover is concerned.
There is also an optimum minimum arc length which must be achieved for interruption. As current zero is approached, the arc attempts to track the instantaneous current requirements of the electrical circuit that is joined to the interrupter. The arc lags thermally because it has more thermal energy and conductants that are necessary to meet the instantaneous current requirements. Ideally, an open interrupter presents an infinite impedance to the circuit and prevents current flow. The real interrupter, on the other hand, presents a finite resistance to the rising recovery voltage being impressed across its terminals. The recovery voltage represents stored energy in the circuit and supplies current to the residual conductivity of the arc. A race thus develops in which the interrupter tries to deionize the arc (by the flow of gas) while energy is being replaced in the form of heat (as a result of I.sup.2 R losses in the arc). If the power input exceeds the power that the interrupter can remove from the arc, the arc regains its conductivity and causes what is known as "thermal failure." Should the arc deionize, the current will decrease and finally extinguish. Another race develops but, instead of a thermal race of energy balances, it is a race between the interrupter recovering its dielectric strength faster than the recovery voltage can rise. Even though the current is out, for all practical purposes, the residual plasma is still very hot and has not achieved its full dielectric strength. SF.sub.6 gas, when used as the arc extinguishing agent, rapidly recovers its dielectric strength and prevents the flow of charged particles necessary for breakdown or "restrike."
In summary, the arc becomes ionized due to the heat supplied from the circuit. The arc must be cooled very rapidly as current zero is approached if the space occupied by the arc is to become a high-resistant insulator. Moreover, if the arc is to remain deionized, the interrupter must remove more energy from the arc than is supplied from the recovery circuit following current zero. Thus, if the arc is too short or too long, the interrupter will either generate, respectfully, excess arc energy or excess dielectric stress.
Puffer interrupters, for the most part, are designed such that the arc is drawn down into a nozzle-like contact into a probe-like protrusion (See Kucharski, U.S. Pat. No. 3,946,180). An efficient design allows the arc to achieve near optimum length in a very short period of time, much as that found in multi-pressure circuit breakers. Milianowicz (U.S. Pat. No. 3,331,935) teaches several embodiments of a gas blast circuit breaker having a dual piston arrangement to provide a so-called "double-acting" puffer interrupter. Simply stated, two pistons are driven towards each other to maximize the rate of gas compression. One of the pistons is driven home by a cocked spring. Yoshioka (U.S. Pat. No. 3,745,281) is similar to Milianowicz. Yoshioka uses an electro-magnetic force generated between a primary coil, which is fixed to the operating rod which moves one of the contact elements, and a ring fixed to a slideably supported puffer piston.
It is also common in the design of conventional puffer interrupters to produce an arc before the nozzle, where interruption is to take place, is physically opened to allow the flow of interrupting gas (for example U.S. Pat. No. 3,941,963 issued to Sasaki). McConnell (U.S. Pat. No. 3,914,569) disclosed a puffer interrupter having a moving assembly connected to the nozzle of the puffer interrupter that is extended and contracted in response to the stroking of one piston and having a second piston-like surface formed in the cylindrical body of the puffer interrupter to control the location at which arcing occurs. In accordance with that invention, a moveable contact and a moveable nozzle are connected to each other such that the moveable contact is repositioned downstream the nozzle throat at the time an arc is drawn between the two contacts. While contamination of the nozzle throat is reduced, the total energy release of the gas blast, especially during the early part of the opening stroke, is also reduced where, of course, interruption cannot be affected.
Kramer (U.S. Pat. No. 3,671,698) uses a moveable contact member carrying a dual piston structure to dampen the opening movement of the moveable contact as well as the closing movement. It functions much as an ordinary dash pot. Korner (U.S. Pat. No. 3,985,988) disclosed one embodiment of a circuit breaker assembly having a pair of contact elements, one of which is displaced by the pressure occurring within the quenching chamber surrounding the arc which is stuck upon separation of the contact elements. Roston (U.S. Pat. No. 3,987,262) teaches a puffer interrupter having a composite piston structure which is retracted during operation of the interrupter. The result is the production of higher pressures early in the stroke of the puffer piston. Milianowicz (U.S. Pat. No. 3,331,935) is similar with the exception that two opposed puffer pistons are used. It is also common in many puffer interrupters to initiate gas flow before the interrupter is capable of performing interruption. That is, the gas is emmited too soon. This is wasteful and ultimately requires that the prime mover compresses more gas than what is actually required under optimum conditions.
In summary, since the gas must be compressed prior to interruption, it should be clear that successful interruption cannot take place prior to the time that the gas is compressed to the required minimum pressure. It is also fundamental that a smaller volume of gas can be compressed to the required minimum pressure sooner and much more easier than a larger volume of gas. Finally, if the pressure produced across the arc is greater than what is required towards the end of the arc extinguishing cycle, when the arc is almost extinguished, then the excess pressure produced is equally wasteful and indictive of using the prime mover at less than optimum efficiency. A puffer interrupter that is designed to incorporate these fundamental considerations would go far towards achieving the interrupting efficiency heretofore experienced by multi-pressure circuit interrupters.